I think the picture is enough.

Mixing open and close tags like that will lead to undefined/unexpected behavior. I don't think this is a problem Cemetech can or should fix. Keep the open and close tags for each style in the same scope and you won't have any issues.

It might be because of how you put the italics and bold half embedded into eachother weirdly, might be tripping up bbcode!

Yeah exactly what TLM said he just beat me to it...

To further elaborate, bbcode has pretty ill-defined semantics that are usually just "directly translate tags to an HTML equivalent". We currently translate both bold and italic blocks to HTML <span>s with appropriate styling applied, so it happens that a closing tag for either one will actually close the most-recently-opened span as long as there was a matching open tag preceding it.

I did some work on this sort of problem a while ago, which may at some point become the implementation in use on the site.